This application relates generally to gas turbine engine rotor assemblies and, more particularly, to bearing assemblies for gas turbine engine rotor assemblies.
Gas turbine engines typically include a fan rotor assembly, a compressor, and a turbine. The fan rotor assembly includes a fan that includes an array of fan blades extending radially outward from a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan, and is supported longitudinally with a plurality of bearing assemblies. Bearing assemblies support the rotor shaft and typically include rolling elements located within an inner race and an outer race.
Additionally, at least some known bearing assemblies include a plurality of identical springs attached to the bearing outer race. The springs are spaced equally in a single row circumferentially around the rotor shaft to provide radial stiffness to the bearing and to center the outer race with respect to the support frame. A first end of the springs is attached to the bearing assembly outer race, and a second end of the springs is attached to a flange coupled to a support frame.
During operation, an unbalance within the engine may cause the engine rotor shaft to displace radially. The radial displacements of the shaft are transmitted to the bearing assembly. Because the springs are arranged in parallel, the deflection of each spring is the same. A parallel configuration facilitates optimizing an overall weight of the bearing assembly by utilizing a shorter axial space than other configurations. However, the parallel configuration also reduces the spring bending stresses, thus increasing fatigue life of the bearing assembly. In addition, in this stress field, a generic single row configuration would require more springs, a greater material strength, greater cross-sectional inertia, and/or an increased spring length. As a result, a cost and weight of the bearing assembly would increase.
In an exemplary embodiment, a rotor assembly for a gas turbine engine includes a bearing centering sub-assembly that facilitates reducing radial forces transmitted to a bearing assembly. The bearing assembly supports a rotor shaft with a rolling element positioned radially inward from an outer race. The bearing centering sub-assembly is coupled to the outer race and includes a plurality of first springs and a plurality of second springs arranged in a single row that extends circumferentially around the rotor assembly rotor shaft. Each first spring is between adjacent second springs and is coupled between the outer race and an annular ring. Each second spring is coupled between the annular ring and an engine support frame.
During normal operation, radial forces induced to the support frame are minimized by a bearing damper system consisting of an oil plenum and springs. The oil plenum is formed by a radial gap between the outer race and the support frame. A stiffness of the springs determines a rotor natural frequency, whereas the oil plenum controls a frequency response or radial deflection. The springs are beams arranged circumferentially around the rotor shaft, and center the outer race with respect to the support frame to permit the oil plenum to exist.
A cross-sectional inertia, a material modulus of elasticity, a length, and a quantity of beams determines a spring stiffness. The spring intrinsic stiffness is predetermined to allow the rotor to deflect with respect to the support frame such that the oil plenum can dampen radial forces transmitted to the support frame. Because the first spring and the second spring are arranged in a dual parallel configuration, the quantity of springs is doubled while the length of the springs is approximately half that of a single row configuration having the same spring stiffness. Individual spring bending stresses are a function of length. As a result, the parallel configuration reduces bending stresses by approximately fifty percent in comparison the single row configuration. In addition, the parallel configuration also increases fatigue life in comparison the single row configuration.
During high rotor unbalance, if the outer spring deflection is significant, the outer spring may bottom the radial gap in the oil plenum. A circumferential force is created on the outer race springs, yielding the beams in bending. Because the springs are in a parallel configuration, all springs are reduced in length an equal amount, resulting in zero axial translation of the rollers on the inner race. Additionally, a small gap between each first spring and second spring closes and to function as an anti-rotation device that prevents the springs from twisting.